The invention concerns a method in the drying section of a paper machine wherein a so-called single-wire draw is applied and wherein blow boxes are used, which are supposed to prevent phenomena that disturb the mutual support contact between the web and the drying wire, in particular formation of positive pressures in the nips defined by the drying wire and a cylinder face, which blow boxes extend across the entire width of the web, communicate with members that supply blow air, and are provided with at least two nozzle slots or the equivalent transverse to the running and longitudinal direction of the web as well as with a carrier face between the nozzle slots, the carrier face together with the drying wire defining a space.
The invention further concerns an apparatus intended for carrying out the method in accordance with the invention in the area of single-wire draw in the drying section of a paper machine, the device comprising blow boxes, which are fitted substantially across the entire width of the web and which communicate with members which supply blow air, and which blow boxes are provided with at least two nozzle slots or the equivalent transverse to the running and longitudinal direction of the web.
The present invention is applied in connection with a so-called single-wire draw, which, in the present application, means a method of drawing the web over heated drying cylinders in a cylinder group wherein the web runs from one line of cylinders to the other while being supported by one and the same drying wire or fabric so that on one line of cylinders the web is between the drying wire and the cylinder face, and on the other line of cylinders the web is outside the cylinders and the drying wire is between the cylinder face and the web, and the web runs along the draws between the lines of cylinders as supported by the drying wire. It is an advantage of this single-wire draw that the web is continuously supported by the drying wire, and the web has no, or at least no substantially long, open draws, whereby the risk of wrinkles in, and breaks of, the web is reduced.
It is well known that a thin layer of air follows and remains in contact with a moving face so that no gliding takes place between the air and the moving face, but those particles of air that are in contact with the moving face travel at the same speed as the face itself.
In the following, the gap defined by the fabric, such as the drying wire, and by the cylinder or roll face in drying sections in paper machines is called the inlet nip when the fabric arrives in this gap, and the outlet nip when the fabric departs from the gap. If all the faces that define the nip are impenetrable by air, air flows of opposite directions in relation to the gap between the boundary layers are formed both in the inlet nip and in the outlet nip. Thereby, in the inlet nip, owing to the damming effect of the boundary layer flows, a positive pressure is produced across the fabric, and in the outlet nip, owing to the suction effect of the boundary layer flows, a negative pressure is produced across the fabric.
As is known from the prior art, when fabrics penetrable by air, such as wires, are used, the differences in pressure across the fabric, produced by the boundary layer flows, usually produce detrimental air flows through the fabric.
Several prior-art pocket-ventilation devices in paper machines are based on the pumping effect of open drying wires.
As is known from the prior art, the first and the second drying groups in a paper machine are usually provided with said single-wire draw, which is frequently accomplished so that on the upper cylinders the paper web is between the wire and the cylinder, and on the lower cylinders the web is on the wire.
In the Applicant's FI Patent 69,332 (corresponding to U.S. Pat. No. 4,628,618), a device is described that is intended for use in the drying section of a paper machine to prevent phenomena that disturb the mutual support contact between the web and a fabric, such as the drying wire, in particular formation of positive pressures in the gaps or nips defined by the wire and by the cylinder face. This device essentially comprises a blow box extending across the entire width of the web and communicating with members that produce blow air, and said blow box is provided with at least two nozzle slots or the equivalent transverse to the running and longitudinal direction of the web. In the FI Patent 69,332 it is considered novel that, in the device, between said nozzle slots, there is a planar carrier face, whose distance from the fabric facing it is about 10 . . . 30 times, preferably about 15 . . . 25 times the width of said nozzle slot, as well as that, in the space between said plane carrier face and the fabric placed facing it, negative pressure is produced primarily or exclusively by means of the ejection effect of air jets blown through the nozzle slots.
In recent years, the running speeds of paper machines have been increasing constantly, and now a speed of 1500 m/min is being approached. In such a case, fluttering of the web and its detaching from the support fabric become a serious problem detrimental to the running quality of a paper machine.
The threading of the web has become a particularly difficult problem in the area of single-wire draw and especially in connection with the blow boxes or their equivalent described in the FI patent 69,332. The difficulties in threading can be such a bottleneck in the process as to present increasing of the speed of a paper machine to about 1500 m/min or above. In connection with blow boxes in accordance with the FI patent or its equivalent, the threading of the web is made more difficult by the large blow quantities at the service side of the paper machine and by the blowings from the lateral nozzle at the service side, which tend to carry the leader, which has been cut-off from the web, towards the side and thereby cause failures in the threading of the web, which failures substantially increase the time taken for the web to pass through the paper making machine.
The drying wire used in the area of single-wire draw also makes the use of a system of threading ropes more difficult, mainly because the threading ropes must be placed outside the widths of the normal web and wire. In such a case, in connection with threading, the leader band must be detached from the drying wire and be shifted laterally in the area of the threading ropes. Such lateral shifting is particularly problematic in paper machines in which the draw of the web is fully closed from the press section to the dryer.
With the use of prior-art lateral nozzles of blow boxes which keep the web in contact with the wire also in the lateral areas, there has been a problem that, when the blowing of the lateral nozzle placed at the service side of the machine is directed in the plane of the wire towards the threading ropes, the blowing detaches the leader from the threading ropes.